1. Technical Field
The present invention relates to a dielectric barrier discharge lamp lighting device provided with a driving circuit that includes a transformer.
2. Description of the Related Art
In recent years, along with the development of liquid crystal techniques, liquid crystal displays have been generally used as information display devices such as televisions and monitors. The liquid crystal display has a structure in which a light source device (backlight) is placed on the back face of the liquid crystal and a light from the backlight transmits through a liquid crystal panel to provide a screen display. With respect to the light source mainly used for the backlight, a light source constituted by a number of capillary cold cathode fluorescent lamps arranged therein has been used in most cases.
Here, research and development efforts on a dielectric barrier discharge lamp have been vigorously made so as to apply this to the backlight for liquid crystal. The dielectric barrier discharge lamp contains no mercury inside the lamp, and utilizes light emission of rare gas and therefore, this lamp is environment-friendly and superior in recycling property. Moreover, since the dielectric barrier discharge lamp contains no mercury, there is substantially no time-wise change in luminous flux that occurs in a conventional cold cathode fluorescent lamp during a period of time in which mercury inside the lamp has been warmed and sufficiently vaporized. Hence the lamp has the advantage that a quick start-up of light is available.
As one preferable example of a lighting device for a dielectric barrier discharge lamp, a structure that includes a transformer inside thereof, as shown in FIGS. 10A and 10B, has been developed (see JP-A-2004-95379). This lighting device is connected to an external electrode of a dielectric barrier discharge lamp 1 on the secondary coil L2 side of a transformer T. The primary coil of the transformer T is divided into two coil portions L11 and L12. The primary coils L11 and L12 are connected to switching elements S1 and S2 through one-directional current elements D1 and D2.
These switching elements S1 and S2 are alternately turned on and off by a driving signal from a driving circuit 4. As shown in FIG. 10A, when the driving signal from the driving circuit 4 turns S1 on while turning S2 off, a current flows from the primary coil L11 to ground (GND) through the one-directional current element D1 and the switching element S1. At this time, a positive rectangular wave high voltage is generated in the secondary coil L2 to allow a positive pulse current to flow through the lamp 1. Moreover, as shown in FIG. 10B, when the driving signal from the driving circuit 4 turns S1 off while turning S2 on, a current flows from the primary coil L12 to ground (GND) through the one-directional current element D2 and the switching element S2. At this time, a negative rectangular wave high voltage is generated in the secondary coil L2 to allow a negative pulse current to flow through the lamp 1.
The above-mentioned operations allow the dielectric barrier discharge lamp 1 to light up with pulses of the rectangular wave voltage. Here, it is assumed that since an overshoot and an undershoot of the lamp current (current in the secondary coil L2) are suppressed by functions of the one-directional current elements D1 and D2, no unnecessary ripple current flows through the transformer T so that the loss in the transformer T is effectively suppressed.
Moreover, in one example of the transformer T, it is disclosed that the leakage inductance is preferably set to 2.5% or less, with the lamp voltage (output voltage of the transformer) being preferably set in a range from 1000 to 3000 V (see JP-A-10-289791). It is also disclosed that the luminance is improved by 30% through the above arrangement upon lighting up with the same input power.
Moreover, as shown in FIG. 11, it is disclosed that a stray capacity of the secondary coil of the transformer T and a stray capacity generated around the lamp 1 are utilized as one portion of the resonance circuit, while the resonant frequency of the secondary circuit is adjusted, so that the circuit can be miniaturized and becomes more efficient (see JP-A-07-67357). In the circuit shown in FIG. 11, a resonance circuit is constituted by the leakage inductance of the transformer T, an auxiliary capacitor 5 connected in parallel with the lamp 1 and a stray capacity 6 generated in the secondary circuit such as a stray capacity of the transformer T and a stray capacity generated around the lamp 1. It is disclosed that this arrangement supplies a high voltage to the lamp 1, and that the power factor is improved so that the circuit efficiency is consequently improved.